1. Field of the Invention
The invention herein relates to a system and method for catalytically treating a gas stream, and particularly to a system and method for catalytically reducing the content of undesirable compounds in a flue gas resulting from the combustion of fuel.
2. Description of the Related Art
Catalytic treatment for modifying the composition of gas streams is well known in the art. Various types of catalyst beds have been used such as fixed beds and fluid beds. Among fixed beds there are axial flow reactors, radial flow reactors and parallel flow reactors.
Environmental concerns and new regulations are motivating further research and development in the treatment of gas streams to reduce the content of pollutants such as nitrogen oxides (NOx) and sulfur oxides (SOx) from the exhaust gases resulting from combustion processes such as engine or turbine exhaust gases, or furnace stack gas. Such gases can result from the operation of power plants, thermal cracking furnaces, incinerators, internal combustion engines, metallurgical plants, fertilizer plants, chemical plants, and other industrial operations.
For example, processes for selectively reducing the NOx content of flue gas are known. Generally such processes employ the reaction of NOx with a reducing agent, such as ammonia or urea, optionally in the presence of a catalyst. The reduction of NOx with ammonia can be performed catalytically at a temperature generally ranging from about 500° F. to about 950° F. in a process known as selective catalytic reduction (“SCR”).
One problem associated with the catalytic treatment of large volumes of gas is providing an even distribution of the gas across the face of the catalyst, and an even mixing of the flue gas with the reducing agent. As can be readily appreciated, uneven distribution reduces the efficiency of the system. Reduced efficiency, in turn, requires the use of larger catalyst beds.
The weight and bulk of the equipment necessary to achieve satisfactory removal of NOx is yet another important factor. Many industrial plants need to be retrofitted with NOx removal (“deNOx”) or SOx removal (“deSOx”) equipment in order to meet the requirements of more stringent government regulations. However, because of the physical bulk of the deNOx/deSOx systems, the flue gas must be diverted to ground level for treatment and then sent back to the stack for subsequent exhaust to the atmosphere. It would be advantageous to provide a relatively lightweight deNOx and/or deSOx unit which can be incorporated directly into the stack. It can readily be appreciated that better system efficiency, and the consequent reduced bulk of the deNOx and/or deSOx system, is advantageous in providing a stack reactor system suitable for mounting to a stack.